Organic EL (OLED) display devices formed by arranging organic EL (OLED) elements in a matrix are conventionally well known. Among such displays, it is widely expected that a major stream of development of thin-shaped display devices will involve active type OLED display devices, in which the drive current for each OLED element is controlled by a transistor formed for each pixel.
FIG. 1 shows an example of a pixel circuit of a conventional active type OLED display device. The source of a pixel driving P channel TFT 1 is connected to a power source PVdd, and the drain of the TFT 1 is connected to the anode of an organic EL (OLED) element 3, while the cathode of the OLED element 3 is connected to a negative power source CV.
The gate of the TFT 1 is connected to the power source PVdd through an auxiliary capacity C, and further is connected to a data line Data, to which a voltage based on pixel data (luminance data) is supplied, through an n channel TFT 2 for selection. Then, the gate of the TFT 2 is connected to a gate line Gate extending horizontally.
During display, the gate line Gate is raised to an H level, and the corresponding TFT's 2 are turned on. In this state, pixel data (or an input voltage based on the pixel data) is supplied to the data line Data, and the pixel data is charged in the auxiliary capacity C. Then, the TFT 1 is driven by a voltage according to the pixel data, and a current flows to the OLED element 3.
Here, although the amount of light emission of the OLED element 3 and the amount of current are almost strictly proportional, the TFT 1 only permits a current to flow when a potential difference Vgs between the gate of the TFT 1 and the power source PVdd exceeds a predetermined threshold voltage Vth. Accordingly, a voltage (threshold voltage Vth) is added to the pixel data supplied to the data line Data so that the drain current may begin to flow near a black level of an image. Moreover, the amplitude of an image signal by which the luminance of a displayed image becomes predetermined luminance near a white level is given as the amplitude of an image signal.
FIG. 2 shows an example of the relations (V-I characteristic) of input voltages (Vgs), and the luminance of the OLED element 3 and currents icv flowing through the OLED element 3. As shown in FIG. 2, the OLED element 3 is set to begin to emit light when the input voltage Vgs is the threshold voltage Vth, and to emit light of a predetermined luminance when the input voltage corresponds to the white level.
As noted, an OLED display device is composed of a display panel on which many pixels are arranged in a matrix. Consequently, the threshold voltage Vth and the inclination of the V-I characteristic may vary among pixels due to manufacturing defects or tolerances, and the light emission amount relative to a data signal (input voltage) may become uneven among the pixels. Consequently, uneven luminance may be generated. FIGS. 3A and 3B are explanatory diagrams when the threshold voltages Vth or the inclinations of the V-I characteristics of two pixels m and n are dispersed, respectively, and FIG. 3C is an explanatory diagram when the both of them are dispersed. As shown in the drawings, when the threshold voltages Vth are dispersed by a voltage ΔVth in the two pixels, the curves of the V-I characteristics become ones shifted by the voltage ΔVth from each other. Moreover, when the inclinations of the V-I characteristics are dispersed in the two pixels, the inclinations of the curves of the V-I characteristics differ from each other. Incidentally, the dispersion of the threshold voltages Vth and the dispersion of the inclinations of the V-I characteristics may be generated in just part of a display screen.
Accordingly, a method of measuring the luminance of each pixel to correct all of the pixels or only defective pixels in accordance with correction data stored in a memory has been proposed in, for example, Japanese Patent Laid-Open Publication No. Hei 11-282420.
Moreover, a method of dividing a display area of a display panel including many pixels into small areas, measuring a current in each area, calculating the overall tendency, and calculating a coefficient for correcting the entire display, or for performing a correction of each area has also been proposed in, for example, Japanese Patent Laid-Open Publication No. 2004-264793.
However, with the method disclosed in Japanese Patent Laid-Open Publication No. Hei 11-282420, it is generally difficult to accurately measure the luminance of pixels of a panel including many pixels with a reasonably short time, while, with the method disclosed in Japanese Patent Laid-Open Publication No. 2004-264793, only the dispersion of luminance changing continuously over the entire display area, or the luminance unevenness in specific patterns such as vertical lines or horizontal lines, can be corrected.